Styrene, a raw material for major polymer products such as polystyrene, ABS, SBR and the like, is consumed at more than 30 million tons per year worldwide with an increasing demand at a rate of up to 3%, thereby being one of the representative general-purpose monomer products.
It is well-known in the field of chemistry that styrene can be prepared by dehydrogenating ethylbenzene in the presence of overheated water vapor, i.e., steam on a dehydrogenation catalyst bed in a reactor. Such a styrene preparation process is an endothermic reaction that occurs under a high temperature condition, and thus is regarded to be one of the representative great energy-consuming processes. As for the heat source for the reaction, ultrahigh temperature steam is used, where the steam does not participate in the reaction, but just passes through the reactor. The steam is separated from the products and collected as condensed water from the process by recovering the waste heat and then cooling with cooling water. In the above process, the amount of heat corresponding to latent heat when the steam is condensed to water is lost.
In such dehydrogenation process, a high conversion rate of ethylbenzene and high selectivity to styrene which inhibits the generation of side products such as benzene and toluene is considered to be important. In the process, process parameters affecting to dehydrogenation performance may include reaction temperature, reaction pressure, space velocity, a mixing ratio of steam and the like.
Since the dehydrogenation reaction of ethylbenzene is an endothermic reaction, the higher the reaction temperature, the more the reaction is advantageous. However, when the reaction temperature is excessively high, the selectivity to styrene decreases, and a side-reaction which generates benzene, toluene or the like becomes dominant. Due to rather great amount of reaction heat, the outlet temperature of a reactor is significantly lower than the inlet temperature of the reactor. For compensating the temperature difference, the conventional dehydrogenation process employs multiple reactors, and energy equivalent to the amount lost as reaction heat is provided to the reactors.
Related to this, Korean patent application No. 1998-042067 and U.S. Pat. No. 5,856,605 disclose a method for minimizing heat loss as reaction heat by heating the exterior surface of multiple tube reactors filled with catalyst, by using a heat-transfer medium. U.S. Pat. No. 5,358,698 discloses a method for improving the flowability of the fluid and thus the reactor performance, by attaching a baffle with a specific shape inside the reactor.
SHR (Steam to Hydrocarbon Ratio) is defined as a molar ratio of steam to aromatic compounds introduced to a reactor.
In most reactions, water acts as a catalytic poison, however it is well-known that it plays important roles in the dehydrogenation of ethylbenzene. It is known that steam reacts with K and Fe, generates active sites, supplies latent heat to the endothermic reaction of ethylbenzene, and thus removes deposited carbon. Since it needs lots of energy to maintain steam at the temperature more than 600° C., a process using the minimum amount of energy is preferred. When an excessive amount of steam is used at high temperature, an important active component of a dehydrogenation catalyst, i.e., K (potassium) is dissolved and eluted through a reactor outlet. Such an event has been indicated as a main reason for deactivation of the catalyst [Applied Catalysis A: General 212 (2001) 239].
In this circumstance, current studies have been more focused on the development of a catalyst which can maintain high activity under relatively low temperature steam conditions. Korean patent laid-open No. 2001-0028267 and Korean patent laid-open No. 2001-0028268 disclose a method for preventing a decrease in catalyst activity caused by the loss of K, by artificially injecting KOH into a reactor.
Since the number of the resulting product molecules are more than that of the reactants, the conversion rate in the ethylbenzene dehydrogenation becomes lower as the pressure increases. Therefore, it is desired to operate the process under as low a pressure as possible, however without imparting too much load to the capacity of a compressor. When the pressure is lowered, stability is increased due to a decrease in catalyst coking, and also selectivity to the main product is improved owing to the relatively decreased side-reaction. Consequently, it is considered that pressure reduction is very advantageous in the process.
Korean patent application No. 1990-0017968 and U.S. Pat. No. 5,053,572 describe a multistage ethylbenzene dehydrogenation process in which a fraction of ethylbenzene is fed to a first reactor and the remaining fraction of ethylbenzene together with a product of the first reactor is fed to a second reactor in multistage ethylbenzene dehydrogenation process. The intended effect of divergence of ethylbenzene or steam in these prior arts relates to: improvement in the ethylbenzene conversion rate owing to the modification of the mixing temperature and composition of the ethylbenzene and steam; increase in selectivity to styrene; and extended catalyst life due to prevention of coking generation. In other words, these prior arts do not disclose the effect of productivity and process stability as it is intended in the present invention, by making in improvement related to the position of injecting the feed containing steam and ethylbenzene.
FIG. 3 shows an adiabatic reactor conventionally used in styrene manufacturing, in which the catalyst bed has a cylindrical shape and is supported by a screen. The adiabatic reactor may be used connected in series. When the more reactors are connected in the system, an increased flow rate of the feed is required. To increase in the flow rate of the feed, the catalyst bed should be thinner while the contact area should be increased to reduce the linear velocity of gas and the fluidization of catalyst particles. However, once a reactor is established and arranged in a system, it is nearly impossible to reconstruct the shape of the reactor, and replacement with a new reactor disadvantageously requires great expense.
Moreover, most styrene manufacturing plants currently in operation in the world use a reaction system having 2 to 3 adiabatic reactors connected in series, wherein the reaction system further comprises, as shown in FIG. 1, furnaces and heat exchangers, which makes it difficult to connect additional pipe for ultrahigh temperature use and thus extend the system by adding further reactors. Therefore, in the case of extending a reaction system having two adiabatic reactors connected in series, it is generally known to simply add one additional reactor having greater capacity than that of the existing reactors downstream of the system, after the two existing reactors. This reactor to be added usually has a volume 2-5 times greater than that of the existing two reactors in order to get the most from the extension of the system, i.e., to maximize the productivity. However, in that case, the reactor capacity will be significantly out of the optimal system design at the time of system construction. Further, the increased flow rate will be excessive to the existing two reactors, which were designed suitably for a system having two reactors in series. This will cause fracture of catalysts due to fluidization of catalyst particles as well as uneven deactivation of catalysts due to the difference in the amount of catalyst filling the reactor.
In this respect, no method has been found that uses a divergence of feed material before the present invention, in order to solve the problems occurred when extending a system by adding a reactor so as to improve productivity. Further, it is difficult to specifically select the optimal injection point of a feed material to the system, since a conventional styrene monomer manufacturing system as shown in FIG. 1 is composed of a complex heat exchanging network which includes furnaces and heat exchangers for heat supply and waste heat recovery.
In the meantime, the major operational hindrance in the styrene manufacturing reaction system is the temperature of ultrahigh temperature steam discharged from the furnaces (F-1, F-2 and F-3), which is also referred as Hot-Piping Temperature (HPT). HPT is usually limited to a range of 800-900° C. When the operation temperature is out of this range, the operation is automatically halted, and the temperature range is called an interlock temperature. The temperature range has been determined as above, based on the generation of cracks at the junction part owing to material or thermal stress of a heat exchanger and a pipe through which ultrahigh temperature steam passes. Therefore, the temperature range may vary depending on the material and design of the equipment used in the system. In operation, it is important to decrease the temperature as much as possible. However, with the elapse of time, a reactor filed with catalysts becomes deactivated, which requires the increase in reaction temperature, and thus the temperature is accordingly increased. Therefore, it is important to maintain the temperature as low as possible so as to avoid the temperature going over the limited range at the time of halting the operation due to the end of catalyst life. Moreover, by lowering the temperature, the life of equipment used in the system may be advantageously extended.